1. Field of Invention
This invention relates to current limiting elements for preventing electrical overcurrent and more particularly to current limiting elements which are reuseable.
2. Prior Art
In Japanese Patent Application No. S50-87595 is described a current limiting element for preventing electrical overcurrent including a principal current conducting member made of a low resistance conductive elastomer having a high coefficient of thermal expansion and having a circular cross-sectional shape and a heating/heat insulating member which consists of a high resistance conductive elastomer or resistive coating surrounding the circumference of the conducting member. Since the cross-sectional shape of this current limiting element was completely circular, as shown in FIGS. 1(a) and 1(b), the effect of ambient temperature upon the length of time required for current limiting action to occur was great when the amount of current was relatively large, in the vicinity of 20-30 amperes. Specifically, when overcurrent flowed through the principal current conducting part A, the time required for current limiting action to occur was extremely long in a low temperature environment. Even though the conducting part A was heated to some extent by the instantaneous heat generation arising from the Joule effect, the length of time required for heat transmission increased as the circumference of the current limiting element was approached. Furthermore, since the external air temperature was low, the amount of heat radiation was also great. Therefore, the temperature necessary for interrupting the flow of overcurrent was sometimes not reached, and as a result, the internal temperature became non-uniform so that the length of time required for current limiting action to occur was increased. Furthermore, when the amount of heat radiation was greater than the amount of spontaneous heat generation as a result of overcurrent, the overcurrent was never interrupted.
In addition, another drawback was that in high temperature environments, the length of time required for current limiting action to occur was greatly decreased by a phenomenon just the reverse of that discussed above.